2.2. Selection of Urban Functional Units (UFUs) and Field Surveys

A total of 190 urban functional units (UFUs) were selected according to the study delineated by Zhang et al. (2022) [7]. The UFUs were divided into six primary types (public affairs service districts, industry districts, business districts, residential districts, recreation and leisure districts, transportation, and undeveloped land) and 16 secondary types (Table 1). The wetlands included marine areas and river channels that did not grow plants.

Number of primary and secondary urban function units (UFUs) selected in Haikou.

We used the stratified sampling to delineate plots in Haikou. We drew 154 grids of 650 × 650 m in the built-up area of Haikou. Within each grid, we selected 1–3 UFUs based on their representability and cultural importance [6,8]. A total of 190 UFUs were generated. Three plots of 20 × 20 m with high plant diversity in each UFU were selected to maximize the plant diversity of the entire UFU. Then, five 5 × 5 m shrub plots and five 1 × 1 m herb plots were established in the central four corners of each 20 × 20 m plot [2,19,20,21]. In all plots, we described and recorded the characteristics of all plants (species name, plant height and DBH for each tree; species name, plant height, clump number, and coverage for shrub species; species name, plant height, and coverage for herb species). We also used GPS to determine its longitude and latitude.

We conducted surveys in the same UFUs in May-June and November-December of 2015 and 2021, to ensure that the survey covered most of the flowering and fruiting periods. In a preliminary field survey in June and December, we found about 70% of the plants were flowering or bearing fruit. We used the “flora of China” to identify plant species and standardized plant names according to The Plant List (http://www.theplantlist.org/ (accessed on 15 June 2021)) [22]. If plant species could not be identified in the field, we collected samples to compare them with those in the China Flora Herbarium (https://www.cfh.ac.cn/ (accessed on 13 June 2021)) or consulted experts for identification. We distinguished between natural and cultivated plant species based on the definitions of Cheng et al. (2021) [23]. Natural species were those that grow without human disturbance, while cultivated plants were those that require human maintenance to grow and maintain viable populations.